Question

Sa se determine suma primilor 19 termeni ai unei progresi aritmetice
$(an) n \geqslant 1$
daca
$a1 = - 3$
si ratia
$r = 2$

​

Answer 1

S19=(a1+a19)•nr de termeni/2
a19=a1+18r=-3+18•2=-3+36=33
S19=(-3+33)•19/2
S19=30•19/2
S19=15•19
S19=285